Exploring Stereo 3D Feature Extraction (Part 2)

While the previous post introduced the concept of 3D stereo feature extraction, this post will focus more on specific tools for 3D vector capture.

For urban 3D modeling, manual stereo feature extraction typically involves collecting rooftops (and sometimes even more detailed structures such as rooftop mechanical features) as 3D polygon features. To create solid 3D models, an extra step is taken to extrude the feature down to the ground. There are typically numerous methods for doing this. In PRO600, an ERDAS application that runs in both the Bentley Microstation and PowerMap environments, there are three options for building extrusion via the “Create Building” operation:

• Manually measure the ground height.
• Specify a fixed object height.
• Extend the feature down to the surface of a digital elevation model.

If you have a DEM available, clearly this is the least time-consuming option - because you can extrude a large number of buildings all at the same time. There are also a number of options to consider when performing the extrusion: see below for Object Creation options (click on the image to enlarge):

Create Building and Object Creation Preferences in PRO600

To demonstrate extrusion and object creation, here is a screen capture of a set of extracted building features. These have all been extruded with the aforementioned “Create Building” operation.

Extracted 3D Building Vectors Displayed in Bentley PowerMap

Here is the same feature dataset with an orthophoto backdrop. Notice how the building polygons fall around the base of the building (the true location) and not the tops of the buildings. This is one of the advantages over heads-up digitizing directly from orthophotos. If the building has any lean at all, then tracing rooftop vectors off the ortho will produce building polygons in the wrong location.

Building Vectors and ECW Orthophoto Displayed in Bentley PowerMap

Switching to an isometric view and altering the display mode, you can see the buildings rendered from a perspective view.

Perspective View of 3D Building Vectors in Bentley PowerMap

While extruding features is one thing, the process of capturing them is another. PRO600 features an extensible library catalog containing feature definitions. While a default catalog comes with the software, it can be completely modified or replaced if necessary. Double-clicking on a feature in the catalog shows the feature attributes (color, description, style, etc.) which can then be edited and saved. Collected features can also be selected an modified on an individual basis. For example, you may have a selection set of buildings that you would like to highlight with a different line thickness and color to make them stand out from the rest of the building vectors in the dataset.

PRO600 Library Catalog and Shape Definitions

While the "Building" feature above triggers off the "Collect Squared" Microstation/PowerMap command, it is also possible to switch into arc-collection mode mid-feature to model any curving or circular lines. An increasing number of building structures feature curving corners instead of sharp edges, making tools like this a necessity.

Modeling complex feature can take time, so snapping is also important. PRO600 has a few different snapping options, including 2D, 3D, and a 2D/3D hybrid - where the tool switches between 2D and 3D based on a user-defined tolerance. Here is a look at the snapping preferences, which also show optional modifications such as the ability to cut a vertex or add a new vertex (to the feature being snapped to).

PRO600 Snapping Preferences

Aside from building polygon features, there are also specific tools available for mapping linear features such as roads. For example, a parallel line tool is available to reduce the time required to digitize both sides of a road or freeway feature. Since photogrammetric mapping has been widely used in the engineering community (e.g. State-level Departments of Transportation in the USA) a number of tools have emerged that are particular to high-accuracy, high-throughput mapping applications. Fortunately, these tools also lend themselves to a broad number of other applications, including visualization, 3D city modeling, urban/environmental planning, and much more.

Exploring Stereo 3D Feature Extraction (Part 1)

Manual operator-driven 2D and 3D feature extraction techniques have in use for many years. One of the early methods of getting data into a GIS was from 2D digitization. In addition to tablet digitizing systems, softcopy photogrammetry approaches to vector data have also been available for many years.

Remember this? Click the image for details

The photogrammetric approach, also known as stereoscopic feature extraction, is one step closer to the source of the data than GIS-based digitizing. Instead of compiling feature data from an orthophoto or another source of information, stereo extraction uses the original images along with the triangulation metadata. The triangulation metadata consists of exterior orientation parameters that are required for stereo pair generation. Exterior orientation data could also come from airborne GPS/IMU data – which may be necessary for mapping remote areas where collecting ground control points is not an option.

Stereo feature extraction is inherently 3D. The vector data are measured in X, Y, and Z by manipulating a floating cursor while viewing a stereo pair of left and right images. As a result, the point, line, and polygon feature data all have Z values associated with each vertex. Because GIS developed in a 2D paradigm, many early stereo feature extraction packages were coupled with CAD environments as a platform. Basically a stereo viewer would be added for stereo image viewing and 3D measurement, and the feature data would render in the application's native “viewer”. In more recent times applications have been developed for GIS packages in addition to CAD environments, as well as a number of native applications that do not require platform technology.

Stereo Feature Extraction in PRO600

So why is this important? Data used for GIS analysis needs to come from somewhere. While there are a number of methods used for generating vector data, the demand for 3D vector data appears to be on the rise. One area of current discussion is the format and environment for generating and persisting 3D vectors. Currently the end application tends to determine the format, but this can be challenging for people trying to create one-size-fits-all data products (e.g. if I want to create a textured 3D city model, what format should I use without being locked into one system?). In this respect the development of CityGML is intriguing, as the geometric properties are but one aspect of urban information modeling and are viewed as an inter-related component of a much broader system. But persistence models aside, tools are still required for the original content generation.

More on specific 3D feature extraction tools tomorrow...

3D PDF Generation

I thought I would write a "how-to" post after reading some recent coverage on 3D PDFs. There are a few different software packages that can generate 3D PDFs, but I will cover the workflow using LPS, PRO600 Fundamentals, and Bentley PowerMap. The actual 3D PDF generation is done with PowerMap (also supported in MicroStation). Aside from photogrammetric mapping it is also possible to build a model from guesstimating building dimensions or designing a model from scratch.

To produce the 3D PDF below, I performed the photogrammetric pre-processing in LPS and then extracted 3D building vectors in PRO600 Fundamentals (which runs on top of Bentley PowerMap) via stereo feature extraction. After that, the steps are:

1) From PowerMap set the View extent to the area to be exported.
2) Choose File > Print. This opens the dialog below.
3) Ensure "Plot to 3D" is checked.
4) Choose Settings > 3D Plotting: you may want to change some settings such as the background color and a number of advanced settings.
5) Choose File > Print (from within the Print menu) when you're ready to save out the 3D PDF.



Click on the image below to download the 3D PDF I generated. I didn't get into any sophisticated modeling, but the tools within PowerMap and MicroStation will allow for creation of highly realistic models.



This post also gives me a chance to try out Scribd: check out the 2D PDF I exported from PowerMap as well, with imagery as a backdrop. Personally I like the zoom and pan functionality as well as full-screen mode: a nice alternative to viewing in Acrobat...
Los Angeles Ortho and Vectors See here for some more 3D PDF examples from the Bentley MicroStation V8i page.

KML 3D Buildings in Los Angeles

I thought I would make the buildings I extracted for the 3D City Construction webinar last week available - you can now download them from here.

I'll talk about the tools used to create these another day, but this allows you to take a look at a photogrammetrically-derived quick and dirty city model. It doesn't have all the buildings and they're not all perfect, but it does give you an idea of what's possible. These took me about 8 hours to extract - using anaglyph-mode on my laptop (perhaps less time, I didn't time myself). It would be more efficient to use a proper stereo viewing environment but a laptop works fine for smaller jobs.

Note that if you open the file in Google Earth they'll have the floating effect described here on Thursday... So if you want to follow the workflow described in that post you can grab the terrain from here: both the IMG grid and the RRD pyramid layer.

3D City Modeling

I'll be hosting a webinar on December 9th entitled: "3D City Construction Workflows: From Collection to Presentation". This will be an opportunity for us to explore workflows for planning projects and developing 3D building content data, which can then be fed into various GIS and mapping applications. We'll also take a look at data presentation: methods for visualizing and sharing the data.

3D city modeling is a rather nebulous topic, and there are many ways to create 3D city models. Here are a few different options:

- Automatically extruding 2D building polygons digitized from an ortho, either from a rooftop Z value or guesstimating the height. This is the "quick and dirty" method.
- Manually model each building individually and tag it with an XY location (often guesstimating the spatial dimensions). These models may be based on ground photos, oblique imagery, or a combination.
- Photogrammetrically measure and extrude buildings.
- Photogrammetrically measure and extrue buildings, and then model further detail and add elements such as photo-texture.

The webinar will primarily focus on the 3rd option: using photogrammetric tools to measure buildings. In addition to the actual extraction process, other discussion points such as data capture, data quality, accuracy, and other topics will be outlined as well.

If you're a GIS/photogrammetry/mapping professional, please tune in to one of the scheduled webinars. You can sign up here.